What if I am not good enough?
Well, since they took you for an internship, you clearly meet some of their requirements (have basic knowledge, understanding / know-how on the technology / domain etc) to get started. You're not starting out of blank, you know it yourself or not, you have some capabilities to start with.
Remember, they took you as an intern, where the basic target is to groom you so that you can learn and gather knowledge more on the domain / technology, and start working towards making actual contribution. This is sort of on-job training, where they target to train you.
Will I be the "stupid developer" in the group?
Do you want to be? No.
Remember, during the internship, you'll also be in learning phase (as you did in college), only some different and some new things than you did in college / university. Have faith in yourself and keep learning - if you find the learning interesting, I bet you never have to think of this doubt / question again. :)
What if I fail my tasks?
Gear up, and try again. As I repeated twice before, you're an intern, you're expected to fail and learn. At times, the mentor(s) may set you up to fail (yes, intentionally), so as to teach you how to handle the failure and react to the situation. Remember, not everything (even in life outside the workplace) ends up in success, you should know how to handle a failure, as a professional and as a person.
Last advice: Never be afraid of failure, be afraid of the thought of not to try again.
Best of luck!